1. Field of the Invention
The present invention relates to an adjusting apparatus in a magnetic disk drive and in particular to a vane encoder assembly used for adjustment of optical encoder elements utilized to position a magnetic head with respect to a magnetic disk.
2. Description of the Prior Art
A magnetic disk drive requires placement of a magnetic read-write head across the radial length of a magnetic disc, typically from 31/2" to 8" in diameter with high speed and great accuracy. Typically the magnetic head is mounted on the end of an arm or lever which is rotated through an angular segment of sufficient degree to allow traversal by the magnetic head of the data bearing portion of the magnetic disk. Thus, very small angular displacements of the arm or mechanism carrying the magnetic head must be accurately measured. Optical encoder disks are typically used in the prior art for generating a signal proportional to the angular displacement of that portion of the lever mechanism which displaces the magnetic disc. One such optical encoder assembly is depicted in European Patent Application No. 83 305 785.4 published Apr. 4, 1984 (Bulletin 84/14) and assigned to Quantum Corporation of Milpitas, Calif.
Such optical encoders comprise a photodetector positioned on one side of a fixed reticle. The reticle has a plurality of slots or openings defined therethrough. On the opposing side of the reticle is a movable vane having a multiplicity of lines photolithographically defined thereon. The movable vane is disposed between a light source and the reticle. Movement of the movable vane, which is typically made of glass or other transparent media, causes the photolithographically printed lines to be moved across the openings in the fixed reticle so that the photodiodes are exposed to a series of light pulses, typically having a quadrature relationship with each other, which are a function of the degree of angular movement of the magnetic head across the disk.
The gap distance and parallelism of the movable vane and fixed reticle must be very accurately adjusted in order to insure reliable and accurate operation of the optical encoder assembly.
Such a gap and parallelism adjustment mechanisms have typically involved springs or cantilevered multiple screw adjustments such as shown by Nakao, "Head Adjusting Device", U.S. Pat. No. 4,550,352 (1985).
Alternatively, magnetic heads have been adjusted using a shim apparatus such as generally described in connection with Feldstein et al., "Magnetic Head Adjustment Mechanism", U.S. Pat. No. 4,185,309 (1980).
Spring loader flexure plates have also been utilized to provide sensitive adjustments of magnetic transducer heads in tape transports such as shown by Kolm, "Flexure Mounting Plate", U.S. Pat. No. 3,154.372 (1964).
However, each of these prior art devices either involves the adjustment of more than one element, usually through a screw-type adjustment. Furthermore, it is exceedingly difficult with prior art devices to provide for fine adjustments of gap widths while maintaining parallelism between the two surfaces whose gap width is being adjusted. Gap width and parallelism could be adjusted only by a tedious and highly skilled reiterative adjustment process utilizing an adjustment mechanism which was delicate, expensive to manufacture, and which poorly retained the set adjustment.
Therefore, what is needed is some type of mechanism whereby gap width or spacing adjustments may be made in a manner such that parallelism between the adjusted parts is preserved, which is inexpensive to manufacture, and which is rugged enough to maintain the fine adjustment under relatively rough handling.